This invention relates to a signal processing circuit especially suitable for use in a colour television receiver.
In a colour television receiving set, in order to correctly demodulate the hue of a colour signal by a chroma signal processing circuit a hue adjusting circuit is provided so as to establish a correct phase relationship between the chroma signal and the subcarrier reference signal. The hue adjusting circuit is generally constructed such that the correct phase relationship is established usually at the center of the phase variable range. When such processing circuit is incorporated into an integrated circuit, two signals are phase shifted by different phase shifting circuits and then the two dephased signals are combined by a mixing circuit which varies the relative amplitude of the two input signals by a control voltage, to thereby vary the phase of the composite signal.
If the constituent elements of the phase shifting circuit have process variations, the phase and amplitude of the resulting signal are also different from desired values so that when such phase shifted signals are combined together, the phase of the composite signal would also vary. Even in the absence of such process variations it is difficult to obtain a signal having an optimum phase due to the phase shift errors caused by the chroma signal path, burst signal path and subcarrier signal path, etc. As a consequence, even if the control voltage is set at the center of the variable range it is impossible to provide a correct and optimum demodulation phase. In order to obtain the optimum phase it is necessary to decrease as far as possible the variations of the signal processing circuit and the hue adjusting circuit from the prescribed design. Since the former variation is caused by the signal processing circuit it is not easy to correct it, but the latter variation can be corrected relatively readily. For this reason, it has been the usual practice to decrease the phase shift variation, by decreasing the process variations of the component elements that are used to construct the phase shifting circuits. To decrease the variations the elements should be manufactured at high accuracies. However, this measure increases the manufacturing cost. High accuracy of manufacturing of the component elements renders it difficult to incorporate them into an integrated circuit. Accordingly, it has been obliged to construct the phase shifting circuits on the outside of the integrated circuit and the mixing circuit is incorporated into the integrated circuit. Even in such a case, since there are a number of external component elements, it is necessary to provide a number of terminals for the integrated circuit.
Generally in a colour television receiving set the appropriate demodulation of the chroma signal is made by designing a hue adjusting circuit to operate in a variable phase range by taking into consideration the deviation of the phase shift made by the signal path of the chroma signal processing circuit. Where the variable phase range is wide, it is possible to adjust the demodulation phase to an optimum value. However, the viewer of the colour television receiving set is required to adjust the circuit to obtain the optimum hue. For this reason it is desirable that the variable phase range should be narrow.
Accordingly, it is an object of this invention to provide an improved signal processing circuit especially suitable for processing a colour signal received by a colour television receiving set.
Another object of this invention is to provide a signal processing circuit which makes it possible to readily obtain an optimum hue irrespective of the variations of the characteristics of the circuit elements.
Another object of this invention is to provide an improved colour signal processing circuit capable of setting an optimum modulation phase always at the center of a voltage for controlling the hue adjusting circuit thereby enabling the optimum demodulation phase even when the hue adjusting range is set narrow.
Still another object of this invention is to provide an improved signal processing circuit capable of readily incorporating the circuit elements of a phase shifter utilized in the processing circuit into an integrated circuit thereby eliminating the effect of deviation of the characteristics of the circuit elements.
A further object of this invention is to provide a signal processing circuit which makes it possible to form a plurality of circuit elements having different functions such as capacitors, resistors and transistors on the same semiconductor chip thereby decreasing the number of the connecting pins of the integrated circuit.
According to one aspect of this invention there is provided a signal processing circuit for use in a colour television receiving set comprising an input terminal for receiving a chroma signal, a burst gate circuit for deriving burst signals from said chroma signal and, a first phase shifter for shifting the phase of said chroma signal, a demodulator connected to the output of said first phase shifter, a hue adjusting circuit including a second phase shifter for adjusting the phase of said burst signal and a mixing circuit for mixing said burst signal and the output of said second phase shifter, and means for supplying the output of said mixing circuit to said demodulator for demodulating said chroma signal.
According to another aspect of this invention there is provided a signal processing circuit for use in a color television receiving set comprising a hue adjusting circuit connected to receive a chroma signal and including a phase shifting circuit means for shifting the phase of said chroma signal and mixing circuit means for mixing the chroma signal and the output signal of said phase shifting means, the relative amplitude ratio between the chroma signal and the output signal of said phase shifting means being controlled by a control voltage applied to said mixing circuit; a burst gate circuit for deriving burst signals from the output signal of said mixing circuit; a subcarrier producing circuit responsive to the burst signal to produce subcarrier signals in phase with the burst signal; and a demodulator connected to receive the chroma signal from said phase shifting means and the subcarrier signal from said subcarrier producing circuit for demodulating the chroma signal.